The present invention relates to an electroless gold plating solution, and more particularly, to a gold plating solution which uses a novel reducing agent system which is low in toxicity and very stable.
Many types of reducing agents have been used, as reviewed in Gold Bulletin 8 p.119-126 (1975). These have included hypophosphite, hydrazine, diethylglycine, ammonia, thiourea, borohydride, formaldehyde, dimethylamine borane, aldehyde-amine borane, unstable organogold compounds, and others. None of these baths are truly stable or commercially useful on a large scale. Most have very short effective plating lives, often in the range of hours or a few days.
Most of the most useful commercial electroless gold plating solutions contain cyanide either as a stabilizer, a main complexing agent, or both. The U.S. Pat. No. 4,863,766 is typical, in using a gold cyanide complex, another cyanide compound, and hydrazine as the reducing agent. Another bath has been disclosed which contains gold(I) potassium cyanide, potassium cyanide and a borane compound as main constituents, as disclosed in Plating, 57 (1970), pp. 914-920. This plating solution is said to allow a plating rate of 1 micrometer/hr with some stability. However, since these plating solutions contains a large amount of cyanide ions, there is a safety problem in operation of the solutions and in disposal of waste baths, rinses, and dragout.
An electroless gold plating solution not containing cyanide ions, using gold(III) chloride and hydrazine as main constituents is disclosed in U.S. Pat. No. 3,300,328. When trivalent gold ions are used, the solutions require a larger amount of reducing agent, as compared to the case of using gold(I) potassium cyanide. In addition, the electroless gold plating solution disclosed in U.S. Pat. No. 3,300,328 is so unstable that general gold precipitation occurs in the plating solution in about 2 hours, making it impossible to continue plating.
An electroless plating bath which does not use cyanide is disclosed in U.S. Pat. No. 4,880,464. They state that the purpose of the use of a large amount of cyanide ions in a conventional electroless gold plating solution is to form stable gold complex ions, and thus control the rate of precipitation of gold where a plating solution is used with a strong reducing agent. They presumed that thiosulfate ions are suitable for use as a complexing agent in place of cyanide ions. Based on the presumption, an electroless gold plating solution was prepared by selecting thiourea as a reducing agent having a strong reducing power in a neutral aqueous solution, using a thiosulfate gold(I) complex, e.g., gold(I) sodium thiosulfate as a complexing agent, and adding a stabilizer. They found that the thiourea reducing agent gave a plating rate and a plating solution stability comparable to those of conventional plating solutions containing cyanide ions and borohydride or other strong reducing agents. However, the stability, plating rate, and appearance of these solutions are much inferior to those of the present invention. Thiourea is not an ideal reducing agent, as it is a proven carcinogenic agent in laboratory animals.